Extending the height of a septic tank lid

[Mark C]

We have 2 septic tanks with 1200mm concrete lids that have 3 lifting points.  We wish to extend them up 1m to allow us to level the land.  Can we lift the lids off, fit the concrete extensions and refit the lids?  If the tanks are plastic are they designed for the extra weight?  Any advice would be much appreciated.  Thanks.

[ProDave]

I would be very surprised if the tanks are plastic.  Most likely brick or concrete.  Lift the lids and see.

[nod]

I’ve used a local steel fabricator twice 

[saveasteading]

If old they will be brick. If new then GRP. Dig an exploratory and let us know.

[Mark C]

Thanks for the reply.  I have just lifted the lids and they are GRP, they were only installed about 15 years ago when the property was converted.  What are the options?  Thanks again.

[markc]

Odd for a GRP tank to have a concrete lid but you could take off the lid (assuming you want to reuse it) lay a concrete slab and build off that then put the existing lid back on. If the concrete lid is a disk with an inspection hatch, remove the hatch and put a new plastic chamber on top of the existing disk.